Heat resistant lithium cell

ABSTRACT

The present invention provides a cell that does not impair heat resistant safety and electrochemical characteristics such as a discharge characteristic, and enhances long-period reliability. In the cell of the present invention, a nonaqueous solvent has, among compounds represented by the following general formula (1), at least one solvent having a boiling point of 200° C. or higher, and has, among compounds represented by the following general formula (1), at least one solvent having a boiling point of lower than 200° C.; and the total volume ratio at 23° C. of the compounds represented by the following general formula (1) is 95 to 100 percent of the nonaqueous solvent,
 
X—(O—C 2 H 4 )n-O—Y  (1)
 
(where X and Y are independently an alkyl group (number of carbons: 1-4), and n is 1-5).

BACKGROUND OF THE INVENTION

(1) Field of the Invention

The present invention relates to an improvement of an electrolyte oflithium cells.

(2) Description of the Prior Art

Conventional lithium cells can be used in a temperature environment ofup to approximately 85° C. However, when lithium cells are incorporatedinto electrical components of vehicles (air-pressure gauges for tires,on-vehicle devices of the Electronic Toll Collection system, and thelike), FA (Factory Automation) appliances, and the like, the cells areoften exposed to a harsh temperature environment of over 100 to 150° C.

To enhance productivity, when the cells are incorporated into electronicappliances, the technique of reflow soldering is employed. With thistechnique, a cell temperature reaches, though only temporarily, as highas 200 to 260° C. In view of this, there is a need for highly reliablelithium cells in heat resistivity that do not swell or do notdeteriorate their cell characteristics under such harsh temperatureenvironment.

As a technique to enhance safety of secondary lithium cells, there isproposed a technique in which diethylene glycol dimethyl ether ortriethylene glycol dimethyl ether is used as a main solvent of anelectrolytic solution (Japanese Unexamined Patent Publication No.H1-281677).

As a technique to enhance the discharge characteristic of secondarylithium cells and to impart high temperature resistivity thereto, thereis proposed a technique in which the main solvent of the electrolyticsolution is butyl diglyme (diethylene glycol dibutyl ether), which has ahigh boiling point, and a separator and a gasket used are made ofpolyphenylene sulfide, which is heat resistant resin (JapaneseUnexamined Patent Publication No. 2002-298911).

There is also proposed a technique in which carboxylic acid orcarboxylic acid ester is added in a nonaqueous electrolyte (JapaneseUnexamined Patent Publication Nos. H8-321311 and H9-147910).

However, with the technique disclosed in H1-281677, heat resistivity isinsufficient because the separator and gasket used here are made of lowheat-resistant polypropylene (melting point: approximately 150° C.). Forthis reason, the cells cannot be used in the above fields ofapplication, where a long period of stability against temperatures ofnear 150° C. is required, and also cannot be used in reflow soldering,where a cell is exposed to temperatures of at least 200° C.

With the technique disclosed in 2002-298911, although heat resistivityis excellent, the viscosity of the nonaqueous electrolytic solution ishigh because the main solvent is the highly viscous butyl diglyme(diethylene glycol dibutyl ether). This lowers the ionic conductivity ofthe electrolytic solution, resulting in a poor discharge characteristic.

With the technique disclosed in H8-321311, a cell is provided with anonaqueous electrolytic solution in which at least one solvent of highdielectric constant selected from the group consisting of ethylenecarbonate, propylene carbonate, and butylene carbonate, and1,2-dimethoxyethane are mixed at a volume ratio of 3:7 to 7:3. However,the solvent of high dielectric constant reacts with the negativeelectrode under a condition of high temperature and forms a highlyresistant coating film on the surface of the negative electrode. Thisreaction occurs conspicuously in a condition of high temperature, andsince the solvent of high dielectric constant is contained at a highratio of 30 volume percent or higher, the amount of the formed coatinglayer is excessive. Since internal cell resistance increases due to thiscoating film, the cell cannot be used in the above fields ofapplication, where a long period of stability against temperatures ofnear 150° C. is required, and also cannot be used in reflow soldering,where a cell is exposed to temperatures of at least 200° C.

With the technique disclosed in H9-147910, the nonaqueous electrolyticsolution used is a nonaqueous solvent in which at least one cycliccarbonic acid ester of high viscosity selected from the group consistingof ethylene carbonate, propylene carbonate, butylene carbonate, andvinylene carbonate, and dimethyl carbonate, diethyl carbonate, or methylethyl carbonate are mixed at a volume ratio of approximately 1:1. Here,the same problem arises as the technique of H8-321311. Accordingly, thecell cannot be used in the above fields of application, where a longperiod of stability against temperatures of near 150° C. is required,and also cannot be used in reflow soldering, where a cell is exposed totemperatures of at least 200° C.

SUMMARY OF THE INVENTION

In view of the foregoing and other problems in prior art, it is anobject of the present invention to provide a lithium cell that isexcellent in heat resistant safety (a long period of stability againsttemperatures of near 150° C., and usability for reflow soldering, wherea cell is exposed to temperatures of at least 200° C.) and in adischarge characteristic.

(a) In order to accomplish the above object, a lithium cell according tothe present invention comprises a positive electrode, a negativeelectrode, a separator interposed between the positive electrode and thenegative electrode, and a nonaqueous electrolyte including a nonaqueoussolvent and an electrolytic salt, the cell wherein: the nonaqueoussolvent has, among compounds represented by the following generalformula (1), at least one compound having a boiling point of 200° C. orhigher, and has, among compounds represented by the following generalformula (1), at least one compound having a boiling point of lower than200° C.; and a total volume ratio at 23° C. of the compounds representedby the following general formula (1) is 95 to 100 percent of thenonaqueous solvent,X—(O—C₂H₄)n-O—Y  (1)

(where X and Y are independently an alkyl group (number of carbons:1-4), and n is 1-5).

According to the above construction, among compounds (there are caseswhere they are referred to as ether-based compounds) represented by theabove general formula (1), a compound having a boiling point of lowerthan 200° C. has relatively low viscosity. When this compound isincluded in the electrolytic solution, the conductivity of a lithium ionimproves and thus the internal cell resistivity decreases, making itpossible to improve cell characteristics.

However, since the boiling point of the above compound is lower than200° C., at the time of reflow soldering, where a cell is exposed totemperatures of 200 to 260° C., a portion of the compound volatilizesand thus internal cell pressure increases. This may cause the swellingof the cell. However, according to the above construction, amongcompounds represented by the above general formula (1), a compoundhaving a boiling point of 200° C. or higher is included. Although havinghigh viscosity, this compound is excellent in heat stability and thusoperates to alleviate an increase in internal cell pressure at the timeof reflow soldering, which increase results from the compound having aboiling point of lower than 200° C. As a result, the swelling of thecell-is rendered small.

The above ether-based compound is extremely less reactive to theelectrodes than cyclic carbonate such as ethylene carbonate andpropylene carbonate is, which was conventionally used. As a result, alithium cell excellent in heat resistant safety and a dischargecharacteristic is realized.

Note that the volume mixture ratio in the present specification is thatmeasured under the conditions of 23° C. and 1 atm.

(b) In the lithium cell of a first preferred embodiment according to thepresent invention, among compounds represented by the above generalformula (1), the compound having a boiling point of lower than 200° C.may include 1,2-dimethoxyethane, and a volume ratio of the compoundhaving a boiling point of lower than 200° C. may be 50 to 60 percent ofthe total volume at 23° C. of the compounds represented by the abovegeneral formula (1),

Among the compounds that meet the above general formula (1), thecompound having the lowest boiling point is one in which n is thesmallest and the carbons in X and Y are the smallest as well. That is,among the compounds that meet the above general formula (1), thecompound having the lowest boiling point is 1,2-dimethoxyethane (DME),in which n=1 and X and Y are composed of a methyl group.

The critical temperature of 1,2-dimethoxyethane (DME), which is thecompound having the lowest boiling point, is 258° C., and if the volumeratio of a compound having a boiling point of lower than 200° C. andincluding DME is higher than 60 volume percent of the total volume at23° C. of the compounds represented by the above general formula (1),internal cell pressure becomes excessive at the time of normal reflowsoldering (200-260° C.), even if a compound having a boiling point ofhigher than 200° C. is mixed. This promotes the swelling of the cell. Onthe other hand, if the volume ratio is lower than 50 volume percent,internal cell resistivity increases, and thus the effect to improve cellcharacteristics is insufficient. Accordingly, it is preferable that thevolume ratio be restricted within 50 to 60 percent.

(c) In the lithium cell of the first preferred embodiment according tothe present invention, among compounds represented by the above generalformula (1), the compound having a boiling point of lower than 200° C.may be a compound other than 1,2-dimethoxyethane, and the volume ratioof the compound having a boiling point of lower than 200° C. may be 50to 90 percent of a total volume at 23° C. of the compounds representedby the above general formula (1),

Among compounds represented by the above general formula (1), a compoundthat is other than 1,2-dimethoxyethane and has a boiling point of lowerthan 200° C. has a critical temperature of higher than 260° C., and itsvolume mixture ratio can be rendered higher than when using1,2-dimethoxyethane alone. However, if the volume mixture ratio ishigher than 90 volume percent, internal cell pressure becomes excessiveat the time of normal reflow soldering (200-260° C.), even if a compoundhaving a boiling point of higher than 200° C. is mixed. This promotesthe swelling of the cell. In addition, if the volume ratio is lower than50 volume percent, internal cell resistivity increases, and thus theeffect to improve cell characteristics is insufficient. Accordingly, itis preferable that the volume ratio be restricted within theabove-specified range.

(d) In the lithium cell according to the first preferred embodiment ofthe present invention, the nonaqueous solvent may include, as asubsidiary component, cyclic ester carbonate and/or lactone. The totalof the cyclic ester carbonate and/or lactone is 5 volume percent orlower at 23° C.

The use of cyclic ester carbonate or lactone as a subsidiary componenthas the following advantage. Since this subsidiary component has highstability under a condition of high temperature and has a higherrelative dielectric constant than a compound represented by the abovegeneral formula (1), such subsidiary component operates so that a cyclecharacteristic improves. Accordingly, a cell that is excellent in safetyand a discharge characteristic under an environment of high temperature,and has a high cycle characteristic is realized.

However, as described above, these compounds have the problem ofreacting with the negative electrode under a condition of hightemperature and thus forming a highly resistant coating film. However,since the volume mixture ratio is lower than 5 volume percent or lowerof the nonaqueous solvent, the above drawback is restricted to anextremely low level.

(e) In the lithium cell according to the first preferred embodiment ofthe present invention, the electrolytic salt may be lithium bis(trifluoromethanesulfonyl) imide and/or lithium bis(pentafluoroethanesulfonyl) imide.

The use of imide-based lithium salt as an electrolytic salt has thefollowing advantage. Since these compounds are highly stableelectrochemically and thermally, the electrolytic solution does notdeteriorate when exposed to a condition of high temperature at the timeof reflow soldering. Therefore, with this construction, it is madepossible to provide a cell in which deterioration of a dischargecharacteristic is further inhibited in an environment of hightemperature.

(f) In the lithium cell according to first preferred embodiment of thepresent invention, the nonaqueous electrolyte may include at least onecompound selected from the group consisting of carboxylic acid,carboxylic acid ester (excluding lactone), and carboxylic acid anhydrideat 0.01 to 5 pts. mass in total per 100 pts. mass of the nonaqueoussolvent.

When carboxylic acid, carboxylic acid ester, or carboxylic acidanhydride (there are cases where they are referred to as carboxylicacids and the like) is added as an additive in the nonaqueouselectrolyte, this additive reacts with the negative electrode and formsa highly conductive coating film. This coating film causes to inhibitthe reaction between an ether-based compound and the negative electrodein a condition of high temperature. As a result, an increase in internalresistance caused by reflow soldering is inhibited, further improving adischarge characteristic.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic sectional view of a coin type lithium secondarycell according to the present invention.

FIG. 2 is a graph showing the boiling point of an ether-based compoundin relation to cell swelling and internal resistivity.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

With reference to the drawing, the preferred embodiments of the presentinvention will be described in detail with a lithium secondary celltaken as an example. It should be understood that the present inventionis not to be limited to the following preferred embodiments, and variouschanges, modifications, and alterations can be made herein withoutdeparting from the scope of the invention. FIG. 1 is a sectional viewshowing the entire construction of the cell.

Referring to FIG. 1, a lithium secondary cell according to an example ofthe present invention includes, in a cell outer housing can (positiveelectrode can) 1, an electrode assembly 5 composed of a positiveelectrode 2 having spinel type lithium manganese oxide as an activematerial, a negative electrode having a lithium-aluminum alloy as anactive material, and a separator 4 that separates the electrodes. In theseparator 4, an electrolytic solution is impregnated. This electrolyticsolution is such that lithium salt is dissolved in a nonaqueous solventthat has, among compounds represented by the following general formula(1), at least one compound having a boiling point of 200° C. or higher,and has, among compounds represented by the following general formula(1), at least one compound having a boiling point of lower than 200° C.,wherein a total volume ratio of the compounds represented by thefollowing general formula (1) is 95 to 100 percent. This cell is sealedsuch that the opening portion of the positive electrode can 1 and a cellsealing can (negative electrode cap) 7 are caulked and fixed with theintervention of a ring-shaped insulating gasket 6.X—(O—C₂H₄)n-O—Y  (1)

(where X and Y are independently an alkyl group (number of carbons:1-4), and n is 1-5)

Next, a fabrication method of the lithium secondary cell according tothe present invention is described.

[Preparation of Positive Electrode]

Spinel type lithium manganese oxide (LiMn₂O₄) for serving as apositive-electrode active material, carbon black for serving as aconductant agent, and polyvinylidene fluoride for serving as a bindingagent were mixed at a mass ratio of 94:5:1, respectively. This mixturewas pressure-molded in order to have a disc-shaped positive electrodepellet of 4 mm across and 0.5 mm thick. This positive electrode pelletwas vacuum-dried (at 250° C. for 2 hours) to remove the moisture out thepellet. Thus, a positive electrode was prepared.

[Preparation of Negative Electrode]

The negative electrode cap used here was made of a clad materialcomposed of a stainless plate and an aluminum plate adhered to eachother with the aluminum plate facing inside. A metal lithium plate wascontact-bonded onto the surface of the aluminum plate, which was theinner surface of the negative electrode cap, in order to prepare adisc-shaped negative electrode of 3.5 mm across and 0.2 mm thick. Themetal lithium plate, which was contact-bonded onto the surface of thealuminum plate, has an alloying reaction caused by charging anddischarging after the sealing of the cell, and thus the active materialof the negative electrode is rendered a lithium-aluminum alloy.

[Preparation of Electrolytic Solution]

In a nonaqueous solvent in which 1,2-dimethoxyethane (DME) andtetraethylene glycol dimethyl ether (TeGM) were mixed at a volume ratioof 50:50, 1.0 M (mole/liter) of LiN (CF₃SO₂)₂ for serving as anelectrolytic salt was dissolved to prepare an electrolytic solution.

[Preparation of Cell]

A separator made of a nonwoven fabric of polyphenylene sulfide (PPS) wasplaced on the negative electrode, and the electrolytic solution wasinjected into the separator. Then, the positive electrode was placed onthe separator, and a positive electrode can of stainless was furtherplaced thereover. The positive electrode can and the negative electrodecap were caulked and sealed with the intervention of an insulatinggasket made of polyether etherketone. Thus, a lithium secondary cellwith a cell diameter of 6 mm and a thickness of 2 mm was prepared. Notethat PPS and polyether etherketone are resins of high heat resistance(melting point, PPS: approximately 280° C.; polyether etherketone:approximately 340° C.).

Next, the present invention will be further detailed by means ofexamples.

EXAMPLES 1-44 AND COMPARATIVE EXAMPLES 1-9

Cells were prepared in the same manner as the above embodiment exceptthat the kind and compound ratio of the nonaqueous solvent, the kind andadded amount of an additive, and the kind of the electrolytic salt werechanged as shown in Tables 1-3.

TABLE 1 Composition of nonaqueous solvent (vol. %) Solvent of lowSolvent of high Subsidiary Added boiling point boiling point componentKind of amount Kind of DME DGM DGE TGM DGB TeGM PC EC additive (mass %)electrolyte Example 1 50 50 LiN(CF₃SO₂)₂ Example 2 50 50 LiN(CF₃SO₂)₂Example 3 50 50 LiN(CF₃SO₂)₂ Example 4 25 25 50 LiN(CF₃SO₂)₂ Example 510 40 50 LiN(CF₃SO₂)₂ Example 6 50 50 LiN(CF₃SO₂)₂ Example 7 50 50LiN(CF₃SO₂)₂ Example 8 50 25 25 LiN(CF₃SO₂)₂ Example 9 10 40 25 25LiN(CF₃SO₂)₂ Example 10 50 50 LiN(C₂F₅SO₂)₂ Example 11 50 50 LiPF₆Example 12 50 50 LiBF₄ Example 13 60 40 LiN(CF₃SO₂)₂ Example 14 70 30LiN(CF₃SO₂)₂ Example 15 30 70 LiN(CF₃SO₂)₂ Example 16 70 30 LiN(CF₃SO₂)₂Example 17 90 10 LiN(CF₃SO₂)₂ Example 18 49.5 49.5 1 LiN(CF₃SO₂)₂Example 19 48.5 48.5 3 LiN(CF₃SO₂)₂ Example 20 47.5 47.5 5 LiN(CF₃SO₂)₂Example 21 49.5 49.5 1 LiN(CF₃SO₂)₂

TABLE 2 Composition of nonaqueous solvent (vol. %) Solvent of lowSolvent of high Subsidiary Added boiling point boiling point componentKind of amount Kind of DME DGM DGE TGM DGB TeGM PC EC additive (mass %)electrolyte Example 22 49.5 49.5 1 Ethyl 0.01 LiN(CF₃SO₂)₂ formateExample 23 49.5 49.5 1 Ethyl 1 LiN(CF₃SO₂)₂ formate Example 24 49.5 49.51 Ethyl 5 LiN(CF₃SO₂)₂ formate Example 25 49.5 49.5 1 Ethyl 7LiN(CF₃SO₂)₂ formate Example 26 49.5 49.5 1 Ethyl 10 LiN(CF₃SO₂)₂formate Example 27 49.5 49.5 1 formic acid 1 LiN(CF₃SO₂)₂ Example 2849.5 49.5 1 acetic acid 1 LiN(CF₃SO₂)₂ Example 29 49.5 49.5 1 oxalicacid 1 LiN(CF₃SO₂)₂ Example 30 49.5 49.5 1 ethyl 1 LiN(CF₃SO₂)₂ acetateExample 31 49.5 49.5 1 acetic 1 LiN(CF₃SO₂)₂ anhydride Example 32 49.549.5 1 phthalic 1 LiN(CF₃SO₂)₂ anhydride Example 33 89 10 1 LiN(CF₃SO₂)₂Example 34 89 10 1 methyl 0.1 LiN(CF₃SO₂)₂ acetate Example 35 89 10 1Ethyl 0.1 LiN(CF₃SO₂)₂ formate Example 36 89 10 1 Ethyl 1 LiN(CF₃SO₂)₂formate Example 37 89 10 1 n-propyl 1 LiN(CF₃SO₂)₂ formate Example 38 8910 1 isopropyl 1 LiN(CF₃SO₂)₂ formate Example 39 89 10 1 n-butyl 1LiN(CF₃SO₂)₂ formate Example 40 89 10 1 isobutyl 1 LiN(CF₃SO₂)₂ formateExample 41 89 10 1 n-amyl 1 LiN(CF₃SO₂)₂ formate Example 42 89 10 1isoamyl 1 LiN(CF₃SO₂)₂ formate Example 43 89 10 1 n-butyl 0.01LiN(CF₃SO₂)₂ formate Example 44 89 10 1 n-butyl 5 LiN(CF₃SO₂)₂ formate

TABLE 3 Composition of nonaqueous solvent (vol. %) Solvent of lowSolvent of high Subsidiary Added boiling point boiling point componentKind of amount Kind of DME DGM DGE TGM DGB TeGM PC EC additive (mass %)electrolyte Comparative 100 LiN(CF₃SO₂)₂ Example 1 Comparative 100LiN(CF₃SO₂)₂ Example 2 Comparative 100 LiN(CF₃SO₂)₂ Example 3Comparative 100 LiN(CF₃SO₂)₂ Example 4 Comparative 100 LiN(CF₃SO₂)₂Example 5 Comparative 100 LiN(CF₃SO₂)₂ Example 6 Comparative 50 50LiN(CF₃SO₂)₂ Example 7 Comparative 46.5 46.5 7 LiN(CF₃SO₂)₂ Example 8Comparative 45 45 10 LiN(CF₃SO₂)₂ Example 9

The names of the compounds that are abbreviated in Tables 1 to 3 are asfollows.

DME: 1,2-dimethoxyethane

DGM: diethylene glycol dimethyl ether

DGE: diethylene glycol diethyl ether

TGM: triethylene glycol dimethyl ether

DGB: diethylene glycol dibutyl ether

TeGM: tetraethylene glycol dimethyl ether

PC: propylene carbonate

EC: ethylene carbonate

The concentration of the electrolytic salt is 0.75 M in Examples 33-44,and 1.0 M in the rest of Examples.

The following Experiments 1-4 were conducted using the cells of Examples1-44 and Comparative Examples 1-9. These experiments aimed at studyingthe long-period stability in an environment of high temperature, reflowresistivity, and discharge characteristic after reflow of the cellsprepared above in relation to the solvent composition of the nonaqueouselectrolytic solution and an additive.

[Experiment 1]

Using the cells of Comparative Examples 1-6, a study was conducted onthe reflow resistivity, and internal resistance (IR) after reflow of thecells in relation to the main solvent of the electrolytic solution.

<Reflow Resistance Test>

Each cell was put into a reflow furnace that was set such that thesurface temperature of the cell was kept at 150° C. or higher for 230seconds, 200° C. or higher for 90 seconds, and 250° C. or higher for 40seconds (maximum: 260° C.), and a change in the entire length of eachcell was examined.

<Measurement of Internal Resistance>

Internal resistance to an alternating current of 1 kHz was measured foreach cell.

The results of Experiment 1 are shown in FIG. 2.

The boiling point of each solvent is as follows.

1,2-dimethoxyethane: 85° C.

diethylene glycol dimethyl ether: 162° C.

diethylene glycol diethyl ether: 185° C.

triethylene glycol dimethyl ether: 216° C.

diethylene glycol dibutyl ether: 256° C.

tetraethylene glycol dimethyl ether: 275° C.

From FIG. 2, such a tendency has been found that as the boiling point ofthe main solvent becomes higher, the swelling of the cell at the time ofthe reflow resistance test becomes smaller, and internal resistivityincreases. From the results shown in this figure, it has been found thatsuch a cell is preferable that has low internal resistivity realized bya solvent having a boiling point of lower than 200° C. and has low cellswelling realized by a solvent having a boiling point of higher than200° C.

Note that the data of Comparative Example 1, in which1,2-dimethoxyethane (DME) having a boiling point of 85° C. was used, isnot shown in FIG. 2. The cell of Comparative Example 1 burst due toreflow, and thus it was impossible to measure its internal resistivityand swelling.

[Experiment 2]

Using the cells of Examples 1-17, and Comparative Example 1,2, 6, and 7,a study was conducted on the composition of a compound having a boilingpoint of higher than 200° C. and a compound having a boiling point oflower than 200° C. in a nonaqueous solvent of the electrolytic solution,in relation to the cell swelling, internal resistivity (IR), dischargecapacity, high-rate (50 μA) discharge capacity, and low-temperature(−20° C.) discharge capacity of each cell after the reflow resistancetest. As for the cell that burst at the time of the reflow resistancetest, this experiment was not conducted.

The reflow resistance test and the measurement of internal resistivitywere conducted in the same manner as Experiment 1, and dischargecapacity, high-rate discharge capacity, and low-temperature dischargecapacity were measured under the following conditions.

<Measurement of Discharge Capacity>

After subjected to the reflow resistance test, each cell was charged byapplying them a uniform voltage of 3.0 V for 30 hours. Then, adischarging of 500 kΩ specific resistance was conducted and thedischarge capacity of each cell was measured until cell voltage reached2.0 V.

<Measurement of High-Rate Discharge Capacity>

After subjected to the reflow resistance test, each cell was charged byapplying them a uniform voltage of 3.0 V for 30 hours. Then, a high-ratedischarging of 50 μA was conducted and the discharge capacity of eachcell was measured until cell voltage reached 2.0 V.

<Measurement of Low-Temperature Discharge Capacity>

After subjected to the reflow resistance test, each cell was charged byapplying them a uniform voltage of 3.0 V for 30 hours. Then, adischarging of 500 kΩ specific resistance was conducted in an atmosphereof −20° C., and the discharge capacity of each cell was measured untilcell voltage reached 2.0 V.

The results of Experiment 2 are shown in Table 4.

TABLE 4 50 μA −20° C. Discharge discharge discharge IR Swelling capacitycapacity capacity (Ω) (mm) (mAh) (mAh) (mAh) Example 1 311 0.045 2.681.89 1.01 Example 2 438 0.008 2.62 1.61 0.91 Example 3 425 0.010 2.631.55 0.33 Example 4 387 0.018 2.65 1.82 0.95 Example 5 359 0.020 2.631.78 0.94 Example 6 422 0.002 2.64 1.65 0.89 Example 7 446 0.005 2.621.63 0.92 Example 8 419 0.007 2.61 1.61 0.92 Example 9 383 0.015 2.671.71 0.97 Example 10 501 0.005 2.61 1.58 0.96 Example 11 742 0.032 2.411.03 0.03 Example 12 1033 0.035 2.36 0.98 0.03 Example 13 302 0.050 2.671.91 1.08 Example 14 724 0.152 1.99 1.26 0.52 Example 15 687 0.000 2.581.47 0.11 Example 16 407 0.011 2.62 1.63 0.95 Example 17 359 0.036 2.661.75 0.98 Comparative — burst — — — Example 1 Comparative 266 0.061 2.711.90 1.00 Example 2 Comparative 842 0.000 2.52 1.34 0.07 Example 6Comparative 826 0.000 2.49 1.39 0.08 Example 7

It has been found from Table 4 above that a cell having such excellentcharacteristics as an internal resistivity of 446Ω or lower, a cellswelling of 0.045 mm or lower, a discharge capacity of 2.61 mAh orhigher, a high-rate discharge capacity of 1.55 mAh or higher, and alow-temperature discharge capacity of 0.33 mAh or higher is obtainedeach in Examples 1-9, where such a nonaqueous electrolyte was used thatLiN (CF₃SO₂)₂ was dissolved in a solvent having a mixture of, amongcompounds represented by the following general formula (1), a compound(hereinafter referred to as a low boiling point compound) having aboiling point of lower than 200° C. and, among compounds represented bythe following general formula (1), a compound (hereinafter referred toas a high boiling point compound) having a boiling point of higher than200° C. at a volume mixture ratio of 1:1 (23° C.).

On the other hand, in Comparative Examples 1 and 2, where such anonaqueous electrolyte was used that LiN (CF₃SO₂)₂ was dissolved only ina low boiling point compound, the swelling of the cell was 0.061 mm orit burst. In Comparative Examples 6 and 7, where such a nonaqueouselectrolyte was used that LiN (CF₃SO₂)₂ was dissolved only in a highboiling point compound, high-rate discharge capacity was 1.39 mAh orlower, and low-temperature discharge capacity was 0.08 mAh or lower.Thus, in the above Comparative Examples, it has been found that cellcharacteristics are inferior.X—(O—C₂H₄)n-O—Y  (1)

(where X and Y are independently an alkyl group (number of carbons:1-4), and n is 1-5)

This is considered as follows. A low boiling point compound has highchemical stability and has relatively low viscosity. Therefore, whenthis solvent is included in an electrolytic solution, internal cellresistivity decreases, making it possible to improve cellcharacteristics. However, the above compound has a boiling point oflower than 200° C., and at the time of a reflow resistance test, where acell is temporarily exposed to a temperature of 260° C., the solvent maycause an increase in internal cell pressure and the swelling of thecell. However, a high boiling point compound included in the nonaqueoussolvent is, although having high viscosity, excellent in heat stability,and thus operates to alleviate an increase in internal cell pressure atthe time of the reflow resistance test, which increase results from thelow boiling point compound. As a result, the swelling of the cell isrendered small. Thus, a lithium cell excellent in heat resistant safetyand an excellent discharge characteristic was realized.

On the other hand, when a low boiling point compound was used alone, theeffect to alleviate an increase in internal cell pressure at the time ofthe reflow resistance test was not obtained, since such effect is due tothe mixture of a high boiling point compound. Accordingly, it isconsidered that the cell swelled on a large scale, and in ComparativeExample 1, where 1,2-dimethoxyethane (DME) having a boiling point of aslow as 85° C. was used, the cell burst. When a high boiling pointcompound was used alone, since the viscosity of the high boiling pointcompound itself was high, the conductivity of an lithium ion in theelectrolytic solution is rendered small. Accordingly, it is consideredthat especially under the conditions of high-rate discharging andlow-temperature discharging, cell characteristics became inferior.

From the results of Examples 2, and 10-12, where the solvent compositionwas identical and the kind of the electrolytic salt was changed, it hasbeen found that in Examples 2 and 10, where an imide-based electrolyticsalt (LiN (CF₃SO₂)₂, LiN (C₂F₅SO₂)₂) was used, low-temperature (−20° C.)discharge capacity was 0.91 mAh or higher, while in Examples 11 and 12,where a perfluoro-acid-based electrolytic salt (LiPF₆, LiBF₄) was used,low-temperature discharge capacity was as extremely low as 0.03 mAh.

This is considered as follows. The imide-based electrolytic salt hashigh heat stability and thus the electrolytic solution does notdeteriorate after the reflow resistance test. On the other hand, theperfluoro-acid-based electrolytic salt has low heat stability and thusthe electrolytic solution deteriorates remarkably after the reflowresistance test. The deterioration of the electrolytic solutionseriously affects discharge capacity under a low temperature, which isconsidered to be responsible for the results shown in Table 4.

From the results of Examples 1, 13, and 14, where the electrolytic saltwas identical and 1,2-dimethoxyethane (DME) was used as a low boilingpoint compound and tetraethylene glycol dimethyl ether (TeGM) was usedas a high boiling point compound, and the mixture ratio of the lowboiling point compound and high boiling point compound was changed, ithas been found that in the cells (Examples 1 and 13) in which the volumemixture ratio of DME was in the range of 50 to 60 percent, the swellingof each cell was as low as 0.050 mm or lower, and cell characteristicswere excellent. On the other hand, in the cell of Example 14, where thevolume mixture ratio of DME was 70 percent, the swelling of each cellwas as high as 0.150 mm, and cell characteristics deteriorated on alarge scale.

This is considered as follows. The critical temperature of1,2-dimethoxyethane (DME) is 258° C., and thus causes to remarkablyincrease internal cell pressure in the reflow resistance test, where thetemperature temporarily reaches 260° C. When the volume mixture ratio ofDME was 60 percent or lower, because of the effect to alleviate anincrease in internal cell pressure realized by the mixed high boilingpoint compound (in Examples, tetraethylene glycol dimethyl ether), theswelling of the cell was restricted to 0.050 mm or lower. On the otherhand, when the volume mixture ratio of DME was higher than 60 percent,because the effect to alleviate an increase in internal cell pressurerealized by the mixed high boiling point compound became small, theswelling of the cell became as high as 0.150 mm. In addition, it isconsidered that the swelling of the cell decreased the adhesionproperties of the active material, thus deteriorating cellcharacteristics.

From the results of Examples 2-5 and 15-17, where the electrolytic saltwas identical and a solvent other than 1,2-dimethoxyethane (DME) wasused as a low boiling point compound and tetraethylene glycol dimethylether (TeGM) was used as a high boiling point compound, and only themixture ratio of the low boiling point compound and high boiling pointcompound was changed, it has been found that in the cells (Examples 2-5,16 and 17) in which the mixture ratio of the low boiling point compoundwas in the range of 50 to 90 percent, a low-temperature characteristicwas as excellent as 0.33 mAh or higher, while in Example 15, where themixture ratio of the low boiling point compound was 30 percent, alow-temperature characteristic was as extremely low as 0.11 mAh.

This is considered as follows. Since the low boiling point compoundincluded a compound having a higher boiling point and a higher criticaltemperature than those of 1,2-dimethoxyethane (DME), the increase ofinternal cell pressure was smaller at the time of the reflow resistancetest, where the temperature temporarily reaches 260° C., than when onlyDME was used as a low boiling point compound. Accordingly, even thoughthe volume mixture ratio of the low boiling point compound was 90percent, because of the effect to alleviate an increase in internal cellpressure realized by the mixed high boiling point compound (in Examples,tetraethylene glycol dimethyl ether), the swelling of the cell wasrestricted to 0.036 mm or lower. On the other hand, when the volumemixture ratio of the low boiling point compound was 50 percent or lower,because the mixed high boiling point compound was excessive, theviscosity of the electrolytic solution increased. Accordingly, it isconsidered that especially under the condition of low-temperature,discharge capacity became inferior.

[Experiment 3]

Using the cells of Examples 2, 18-21, and Comparative Example 8 and 9, astudy was conducted on the composition of a compound having a boilingpoint of higher than 200° C. and a compound having a boiling point oflower than 200° C. in a nonaqueous solvent of the electrolytic solution,in relation to the cell swelling, internal resistivity, dischargecapacity, high-rate discharge capacity, low-temperature dischargecapacity, and cycle characteristic of each cell after the reflowresistance test.

The reflow resistance test, the measurement of internal resistivity,discharge capacity, high-rate discharge capacity, and low-temperaturedischarge capacity were conducted in the same manner as Experiment 1 or2, and a cycle characteristic was measured under the followingconditions.

<Measurement of Cycle Characteristic>

After subjected to the reflow resistance test, each cell was charged byapplying them a uniform voltage of 3.0 V for 30 hours. Then, adischarging was conducted at a specific resistance of 500 kΩ until cellvoltage reacted 2.0 V, and the number of cycles where a dischargecharacteristic was 50 percent of that of the first cycle was measured.

The results of Experiment 3 are shown in Table 5.

TABLE 5 Discharge −20° C. Cycle IR Swelling capacity 50 μA dischargedischarge characteristic (Ω) (mm) (mAh) capacity (mAh) capacity (mAh)(number) Example 2 438 0.008 2.62 1.61 0.91 15 times Example 18 5470.003 2.57 0.87 0.76 17 times Example 19 707 0.005 2.49 0.77 0.60 19times Example 20 921 0.001 2.41 0.59 0.38 20 times Example 21 695 0.0002.51 0.78 0.59 19 times Comparative 1733 0.007 2.42 0.11 0.05 24 timesExample 8 Comparative 1908 0.008 2.35 0.10 0.01 25 times Example 9

From Table 5 above, such a tendency has been found that as the addedamount of a subsidiary component (cyclic carbonate) increases, a cyclecharacteristic becomes better, as well as an increase in internalresistivity and a decrease in high-rate discharge capacity andlow-temperature discharge capacity. It has been found that when theadded amount of a subsidiary component was 5 volume percent or lower,internal resistivity increased and a decrease in high-rate dischargecapacity and low-temperature discharge capacity was minimized, thusrealizing a cell having a good cycle characteristic. When the addedamount of a subsidiary component was higher than 5 volume percent,internal resistivity became 1733Ω or higher, and high-rate dischargecapacity became 0.11 mAh or lower and low-temperature discharge capacitybecame 0.05 mAh or lower. Thus, cell characteristics deterioratedremarkably.

This is considered as follows. Cyclic carbonate, used as a subsidiarycomponent, has high stability to high temperature and has a higherrelative dielectric constant than an ether compound, which is a mainsolvent, and thus operates to improve a cycle characteristic. However,cyclic carbonate is highly reactive to the negative electrode and thusforms a highly resistant coating film on the surface of the negativeelectrode. This increases internal resistivity, and thus degrades cellcharacteristics. When the added amount of cyclic carbonate is 5 volumepercent or lower, such a cell is obtained that internal resistivityincreases, a decrease in high-rate discharge capacity andlow-temperature discharge capacity is minimized, and a cyclecharacteristic is satisfactory, which is preferable.

[Experiment 4]

Using the cells of Examples 2, 18, and 22-32, a study was conducted onthe kind of an additive and the added amount thereof, in relation to thecell swelling, internal resistivity, discharge capacity, high-ratedischarge capacity, low-temperature discharge capacity, and cyclecharacteristic of each cell after the reflow resistance test. The reflowresistance test, the measurement of internal resistivity, dischargecapacity, high-rate discharge capacity, and low-temperature dischargecapacity were conducted in the same manner as Experiment 1 or 2.

TABLE 6 50 μA −20° C. Discharge discharge discharge IR Swelling capacitycapacity capacity (Ω) (mm) (mAh) (mAh) (mAh) Example 2 438 0.008 2.621.61 0.91 Example 18 547 0.003 2.57 0.87 0.76 Example 22 459 0.011 2.580.87 0.76 Example 23 420 0.025 2.49 0.81 0.68 Example 24 373 0.041 2.450.78 0.69 Example 25 351 0.061 2.41 0.75 0.65 Example 26 384 0.087 2.380.71 0.59 Example 27 411 0.027 2.53 0.85 0.75 Example 28 405 0.032 2.510.83 0.71 Example 29 382 0.031 2.55 0.88 0.78 Example 30 391 0.028 2.500.85 0.75 Example 31 401 0.022 2.51 0.82 0.72 Example 32 415 0.030 2.480.77 0.66

From Table 6 above, such a tendency has been found that as the addedamount of ethyl formate increases, internal resistivity decreases andthe swelling of the cell increases (Examples 22-26). It has been foundthat when the added amount of an additive was 5 mass percent or lower,such a cell is obtained that the swelling of the cell was minimized andinternal resistivity was low. When the added amount of an additive was 5mass percent or higher, high-rate discharge capacity became 0.75 mAh orlower and low-temperature discharge capacity became 0.65 mAh or lower.Thus, cell characteristics deteriorated remarkably.

This is considered as follows. Carboxylic acid, carboxylic acid ester,and carboxylic acid anhydride (carboxylic acids and the like), used asadditives, form a highly conductive coating film on the surface of thenegative electrode, and thus are able to inhibit the reaction of anether-based compound, which is a main solvent, or propylene carbonate tothe negative electrode. Accordingly, such carboxylic acids and the likeoperate to reduce internal resistivity. However, carboxylic acids andthe like react to a manganese compound included in the positiveelectrode because of reflow, and thus decompose to generate a gas. Thisincreases internal cell pressure and causes the swelling of the cell.When the added amount of carboxylic acids is 0.01 to 5 mass percent,such a cell is obtained that the swelling of the cell is minimized andinternal resistivity is low.

[Experiment 5]

Using the cells of Examples 33-44, a study was conducted on an additivein relation to the pulse discharge characteristic before reflow,internal cell resistivity and pulse discharge characteristic after thereflow resistance test of each cell. The reflow resistance test and themeasurement of internal resistivity were conducted in the same manner asExperiment 1.

<Pulse Discharge Test>

A pulse discharging was conducted for 0.29 seconds at a specificresistance of 3.6 kΩ. Here, the lowest voltage was rendered pulsedischarge voltage.

TABLE 7 Before reflow After reflow Pulse Pulse discharge dischargevoltage Internal voltage Internal (V) resistivity (Ω) (V) resistivity(Ω) Example 33 2.24 122 1.44 920 Example 34 2.28 95 1.63 415 Example 352.30 109 1.94 309 Example 36 2.31 106 1.99 296 Example 37 2.31 104 2.00291 Example 38 2.29 108 1.95 301 Example 39 2.33 101 2.05 283 Example 402.29 107 2.01 295 Example 41 2.29 104 2.03 289 Example 42 2.28 109 1.94302 Example 43 2.28 108 1.98 297 Example 44 2.31 102 2.04 285

It has been found from Table 7 that in Examples 34-44, where an additive(carboxylic acids and the like) was added, pulse discharge voltage afterreflow was 1.63 to 2.05 V and internal resistivity was 283 to 415Ω, thatis, these results were much superior to Example 33, where no additivewas added and pulse discharge voltage after reflow was 1.44 V andinternal resistivity was 920Ω.

This is considered as follows. Carboxylic acid ester, used as anadditive, forms a highly conductive coating film on the surface of thenegative electrode, and thus is able to inhibit the reaction of anether-based compound, which is a main solvent, or propylene carbonate tothe negative electrode. Accordingly, such carboxylic acid ester operatesto reduce internal resistivity. It is considered that the decrease ofinternal resistivity caused to improve a pulse discharge characteristic.

It has been found from a comparison between Examples 39, 43, and 44,where the added amount of n-butyl formate serving as an additive waschanged, that when the added amount was in the range of 0.01 to 5 masspercent, pulse discharge voltage after reflow was 1.98 to 2.05 V, thatis, there was no major difference in pulse discharge voltage afterreflow. Accordingly, when the added amount of n-butyl formate is in therange of 0.01 to 5 mass percent, pulse discharge voltage after reflowimproves sufficiently.

It has been found from Experiments 4 and 5 that when the compound ratioof a low boiling point compound is high, the effect to improve cellcharacteristics realized by the addition of carboxylic acids isexhibited remarkably, although the reason has not been detected.

[Supplementary Remarks]

(1) Carboxylic acids include carboxylic acid such as formic acid, aceticacid, propionic acid, oxalic acid, maleic acid, benzoic acid, phthalicacid, metaphthalic acid, and terephthalic acid; carboxylic acid ester(excluding lactone) such as methyl formate, ethyl formate, n-propylformate, isopropyl formate, n-butyl formate, isobutyl formate, amylformate, isoamyl formate, methyl acetate, ethyl acetate, and methylpropionate; and carboxylic acid anhydride such as acetic anhydride andphthalic anhydride. The use of the foregoing realizes a similaradvantageous effect.

However, if a large amount of carboxylic acids and the like is added inthe electrolytic solution, carboxylic acids, upon exposure to acondition of high temperature such as reflow, react to a manganesecompound included in the positive electrode, and thus generate a gas.This may cause the swelling of the cell. In view of this, the addedamount of carboxylic acids and the like is preferably 0.01 to 5 pts.mass per 100 pts. mass of the electrolytic solution.

To obtain the effect of carboxylic acids and the like sufficiently, thecomposition amount of a low boiling point compound is preferably 50percent or higher, more preferably 60 percent or higher, and furthermore preferably 70 percent or higher.

(2) In the above Examples, although ethylene carbonate and propylenecarbonate was used as a subsidiary component, other cyclic carbonatessuch as butylene carbonate and vinylene carbonate, or lactone such asγ-butyrolactone may be used. Alternatively, a mixture of the foregoingmay be used.

(3) The application of the present invention is not limited to lithiumsecondary cells such as those described in the above examples; it isapplicable to any lithium cells such as lithium primary cells, wheresimilar excellent effects are obtained.

(4) When the present invention is applied to lithium secondary cells, itis preferable to use spinel type lithium manganese oxide (LiMn₂O₄) as apositive-electrode active material because it is low cost and has highheat stability. It is also possible, however, to use otherlithium-containing transition metal oxides such as lithium-containingcobalt oxide (LiCoO₂), lithium-containing nickel oxide (LiNiO₂), andlithium-containing iron oxide (LiFeO₂), or a mixture thereof. It is alsopossible to use lithium-containing transition metal oxides that have inthe crystal lattice thereof other metal elements.

As for a negative-electrode active material, it is preferable to usemetal or the like that metalizes with lithium metal, lithium alloy, andlithium.

(5) When lithium metal or lithium alloy is used for the negativeelectrode, metal oxide such as manganese dioxide and diniobium pentoxidethat does not contain lithium and intercalates and releases a lithiumion may be used. Such metal oxide may be used alone or along with boronoxide contained therein.

(6) When the present invention is applied to lithium primary cells,manganese dioxide, graphite fluoride, iron disulfide, iron sulfide, orthe like may be used as a positive-electrode active material. Manganesedioxide is preferable in terms of heat stability.

As for a negative-electrode active material, it is preferable to uselithium metal, lithium alloy, or the like.

(7) As for an electrolytic salt, it is preferable to use imide-basedlithium salt in terms of heat stability. However, a small amount oflithium salt other than the above may be included.

(8) The cell of the present invention endures over a long period of usein a severe environment of high temperature. For that purpose, theseparator should be made of a material that has a heat-resistanttemperature (melting/decomposition temperature) of preferably over 150°C., more preferably over the melting temperature of reflow soldering(185° C.), particularly preferably over the lowest reflow temperature(200° C.), and most preferably over the highest reflow temperature (260°C.).

The above materials for a separator include, other than theaforementioned polyphenylene sulfide and polyether etherketone, heatresistant resins such as polyether ketone, polybutylene terephthalate,and cellulose, or resins whose heat resistance temperatures are enhancedby adding a filler such as glass fiber in the resin materials.

(9) In the above Examples, in sealing the opening portion of the cellouter housing can, caulking with the use of a gasket was used. However,instead of this technique, laser radiation or the technique of heatingand depositing a sealing member made of heat resistant resin may beused.

When a gasket or heat resistant resin is used for sealing the cell, interms of the heat resistant reliability (prevention of leakage) of thecell, the material of the gasket or heat resistant resin desirablysatisfies the heat-resistant temperature conditions for the material ofthe separator.

As has been described above, the present invention realizes a lithiumcell that is used safely for a long period of time in a high temperatureenvironment of 100 to 150° C. and that inhibits the deterioration ofdischarging performance even in such environment of high temperature.Since such cell of the present invention has excellent heat resistantsafety and an excellent discharge characteristic, when the cell isconstructed, it is possible to employ the technique of reflow soldering,which entails a high temperature of 200 to 260° C., although such hightemperature is required as temporarily as 100 seconds. In this case aswell, upon exposure to reflow heating, there is no swelling of the cellor deterioration of cell performance.

1. A lithium cell comprising: a positive electrode; a negativeelectrode; a separator interposed between the positive electrode and thenegative electrode; and a nonaqueous electrolyte including a nonaqueoussolvent and an electrolytic salt, wherein: the nonaqueous solventincludes: among compounds represented by the following general formula(1), at least one compound having a boiling point of 200° C. or higher;and among compounds represented by the following general formula (1), atleast one compound having a boiling point of lower than 200° C.; and atotal volume ratio at 23° C. of the compounds represented by thefollowing general formula (1) is 95 to 100 percent of the nonaqueoussolvent; the compound having a boiling point of lower than 200° C.includes 1,2-dimethoxyethane; and a volume ratio of the compound havinga boiling point of lower than 200° C. is 50 to 60 percent of a totalvolume at 23° C. of the compounds represented by the following generalformula (1),X—(O—C₂H₄)n-O—Y   (1) (where X and Y are independently an alkyl group(number of carbons: 1-4), and n is 1-5).
 2. The lithium cell accordingto claim 1, wherein the nonaqueous solvent includes cyclic estercarbonate and/or lactone, the cyclic ester carbonate and/or lactonebeing a subsidiary component of the nonaqueous solvent and having avolume of 5 percent or lower of the total volume of the nonaqueoussolvent at 23° C.
 3. The lithium cell according to claim 1, wherein theelectrolytic salt is lithium bis(trifluoromethanesulfonyl)imide and/orlithium bis(pentafluoroethanesulfonyl)imide.
 4. The lithium cellaccording to claim 1, wherein the nonaqueous electrolyte includes atleast one compound selected from the group consisting of carboxylicacid, carboxylic acid ester, and carboxylic acid anhydride at 0.01 to 5parts by mass in total per 100 parts by mass of the nonaqueous solvent,the carboxylic acid ester excluding lactone.
 5. A lithium cellcomprising: a positive electrode; a negative electrode; a separatorinterposed between the positive electrode and the negative electrode;and a nonaqueous electrolyte including a nonaqueous solvent and anelectrolytic salt, wherein: the nonaqueous solvent includes: amongcompounds represented by the following general formula (1), at least onecompound having a boiling point of 200° C. or higher; and amongcompounds represented by the following general formula (1), at least onecompound having a boiling point of lower than 200° C.; and a totalvolume ratio at 23° C. of the compounds represented by the followinggeneral formula (1) is 95 to 100 percent of the nonaqueous solvent; thecompound having a boiling point of lower than 200° C. is a compoundother than 1,2-dimethoxyethane; and a volume ratio of the compoundhaving a boiling point of lower than 200° C. is 50 to 90 percent of atotal volume at 23° C. of the compounds represented by the followinggeneral formula (1),X—(O—C₂H₄)n-O—Y   (1) (where X and Y are independently an alkyl group(number of carbons: 1-4), and n is 1-5).
 6. The lithium cell accordingto claim 5, wherein the electrolytic salt is lithiumbis(trifluoromethanesulfonyl)imide and/or lithiumbis(pentafluoroethanesulfonyl)imide.
 7. The lithium cell according toclaim 5, wherein the nonaqueous electrolyte includes at least onecompound selected from the group consisting of carboxylic acid,carboxylic acid ester, and carboxylic acid anhydride at 0.01 to 5 partsby mass in total per 100 parts by mass of the nonaqueous solvent, thecarboxylic acid ester excluding lactone.
 8. The lithium cell accordingto claim 5, wherein the nonaqueous solvent includes cyclic estercarbonate and/or lactone, the cyclic ester carbonate and/or lactonebeing a subsidiary component of the nonaqueous solvent and having avolume of 5 percent or lower of the total volume of the nonaqueoussolvent at 23° C.